1. Field of the Invention
The present invention relates to a voltage regulator, and more specifically, to an improvement of transient response characteristics when an output current fluctuates.
2. Description of the Related Art
FIG. 6 illustrates a conventional voltage regulator including an output current detection circuit. A differential amplifier circuit 104 compares an output voltage of a reference voltage circuit 103 and an output voltage of a voltage dividing circuit 106 to each other and controls a gate-source voltage of an output transistor 105, to thereby obtain a desired voltage at an output terminal 102. An output current detection circuit 107 includes a detection transistor 112, an output current monitoring circuit 113, and a control circuit 114.
When the output terminal 102 of the voltage regulator decreases because of an increased load current, the differential amplifier circuit 104 operates so as to increase the gate-source voltage of the output transistor 105. The output transistor 105 and the detection transistor 112 are transistors having the same characteristics but different K values, and are current-mirror connected to each other. Therefore, the detection transistor 112 allows a current Im corresponding to a load current of the output voltage 102 to flow. The output current monitoring circuit 113 converts the current Im flowing through the detection transistor 112 into a voltage, and outputs the voltage. In response to the voltage output from the output current monitoring circuit 113, the control circuit 114 generates and outputs a control signal. In response to the control signal output from the control circuit 114, the differential amplifier circuit 104 increases a bias current.
As described above, in the conventional voltage regulator, the output current detection circuit controls the bias current of the differential amplifier circuit 104 in accordance with the load current, and hence transient response characteristics are improved (see, for example, Japanese Patent Application Laid-open No. 2011-96210).
However, the conventional voltage regulator including the output current detection circuit detects the load current by an output signal of the differential amplifier circuit 104, thereby controlling the bias current of the differential amplifier circuit 104. Thus, it has been difficult to swiftly respond to a decrease in output voltage. In other words, there has been a problem in that, when the load current is switched from a light load to a heavy load, the bias current of the differential amplifier circuit 104 is reduced, and hence the transient response characteristics of the differential amplifier circuit 104 at the time of detecting the decrease in output voltage are poor.